slack bridge: Add logic to prevent looping messages.

When using Slack Webhook integration to get messages
from Slack to Zulip, we don't want to send back
messages from the Slack integration bot.

Fixes zulip#825.

zulip/integrations/bridge_with_slack/bridge_with_slack_config.py

@@ -3,6 +3,7 @@
         "email": "zulip-bot@email.com",
         "api_key": "put api key here",
         "site": "https://chat.zulip.org",
+        "integration_bot_email": "slack-bot@zulip.com",
     },
     "slack": {
         "username": "slack_username",

zulip/integrations/bridge_with_slack/run-slack-bridge

@@ -89,7 +89,8 @@ class SlackBridge:
             slack_channel = get_slack_channel_for_zulip_message(
                 msg, self.zulip_to_slack_map, self.zulip_config["email"]
             )
-            if slack_channel is not None:
+            is_from_slack = msg["sender_email"] == self.zulip_config.get("integration_bot_email")
+            if slack_channel is not None and not is_from_slack:
                 self.wrap_slack_mention_with_bracket(msg)
                 slack_text = SLACK_MESSAGE_TEMPLATE.format(
                     username=msg["sender_full_name"], message=msg["content"]